Will ensure that fluids will not siphon from tank to tank. Commonly used in marine toilet and engine cooling systems. The valve system incorporated in the top of the loop allows air to enter the line when not in use yet seals as water or other fluid passes through. Molded in glass-reinforced Marelon neither vent or loop will corrode. The duckbill one way valve is designed to seal properly if particulate matter is present in fluids.